The present invention relates to thin-film protection of visible light photocathodes and more particularly to photocathodes protected by thin films and photon sensors and fast electron sources incorporating such photocathodes.
It is well known that photosensitive materials operative in the visible range are highly reactive with oxygen, moisture and other impurities. It has been proposed to extend the lifetimes of such materials in a low vacuum or gas environment by coating them with thin solid protective films. The task is not simple since on the one hand the film must be as thin as possible so as to transmit photoelectrons from the photocathode, and on the other hand, sufficiently thick to prevent diffusion of undesired molecules from the gas to the photocathode.
Reference is made iii this context to the following publications, the disclosures of which are hereby incorporated by reference:
R. Enomoto, T. Sumiyoshi and Y. Fujita, Nucl. Instrum. and Meth. A343, 117 (1994);
V. Peskov, A. Borovik-Ramanov, T. Sokolova, E. Silin, Nucl. Instrum. and Meth. A353, 184 (1994);
A. Breskin, A. Buzulutzkov and R. Chechik, IEEE Trans. Nucl. Sci. 43, 298 (1995);
A. Buzulutskov, A. Breskin, R. Chechik, J. Va""vra, Nuclear Instruments and Methods in Physics Research A 371 (1996) 147-150;
A. Breskin, A. Buzulutskov, R. Chechik, M. Prager and E. Shefer, Appl. Phys. Lett. 69, 1008 (1996);
A. Buzulutskov, A. Breskin and R. Chechik, J. Appl. Phys. 81,466 (1997).
Protected photocathodes can find applications in photon sensors such as photon detectors, image intensifiers, TV camera tubes and the like. They can also be applied in accelerators, e.g. in intense electron sources inside radio frequency (RFD) guns. In the latter application, photocathodes operative in vacuum emit electron flushes when irradiated with intense fast laser beams. Protected photocathodes simplify installation and operation and increase the lifetime of the devices.
The present invention seeks to provide methods for protecting visible light photocathodes with thin protective films as well as photocathodes protected by thin films and photon sensors and fast electron sources incorporating such photocathodes.
There is thus provided in accordance with the invention a thin film-coated photocathode, including a photocathode formed of a first material consisting of potassium cesium antimonide and a thin film coating of a second material consisting of cesium bromide (CsBr).
The materials potassium cesium antimonide and CsBr used in the present invention have lattice constants which are matched.
For the purposes of this patent application xe2x80x9cmatchedxe2x80x9d or xe2x80x9cmatchingxe2x80x9d means that the atoms of the first and second materials have a spatial relationship therebetween which is periodic. Thus lattice constants that are, for example, identical, or differ by a factor of 2 or the square root of 2 are considered to be matched.
The coated photocathode according to the invention may be transmissive or reflective. Reflective photocathodes may be formed on any clean polished surface. Transmissive photocathodes may be formed on an optically transparent surface, e.g. glass, or on an optical fiber face plate, on a scintillating crystal or on a scintillating fiber face plate.
Additionally in accordance with a preferred embodiment of the present invention there is provided a photon sensor including an electron multiplier and an associated photocathode formed of a first material consisting of potassium cesium antimonide and a coating of a second material consisting of CsBr.
The photon sensor according to the invention may be any photon sensor known in the art such as photon detector, e.g. imaging photon detector, image intensifier and TV camera tube.
The electron multiplier in the photosensor may be any suitable electron multiplier such as a vacuum or gaseous electron multiplier, a wire chamber, an avalanche chamber, a microstrip, microgap, microdot or other micropattem chamber, a Micromegas chamber and a microhole chamber (GEM).
There is also provided according to the invention a fast electron source including a fast photon flux pulse source, an electron accelerator and an associated photocathode formed of a first material consisting of potassium cesium antimonide and a coating of a second material consisting of CsBr. The coated photocathode is preferably thus arranged that it receives as input a fast photon flux pulse from said photon flux pulse source and emits in response a fast pulse of electrons, which are then accelerated by said electron accelerator to provide a fast pulse of energetic electrons.
There is also provided in accordance with a preferred embodiment of the invention a method of providing a photon sensor, more particularly a photon detector, including providing an electron multiplier and providing a photocathode associated with said electron multiplier wherein said photocathode is formed of a first material consisting of potassium cesium antimonide and a coating of thin film of a second material consisting of CsBr.